Rubbers in particular those comprising repeating units derived from isoolefins are industrially prepared by carbocationic polymerization processes. Of particular importance is butyl rubber which is a elastomer of isobutylene and a smaller amount of a multiolefin such as isoprene.
The carbocationic polymerization of isoolefins and its elastomerization with multiolefins is mechanistically complex. The initiator system is typically composed of two components: an initiator and a Lewis acid co-initiator such as aluminum trichloride which is frequently employed in large scale commercial processes.
Examples of initiators include proton sources such as hydrogen halides, alcohols, phenols, carboxylic and sulfonic acids and water.
During the initiation step, the isoolefin reacts with the Lewis acid and the initiator to produce a carbenium ion which further reacts with a monomer forming a new carbenium ion in the so-called propagation step.
The type of monomers, the type of diluent or solvent and its polarity, the polymerization temperature as well as the specific combination of Lewis acid and initiator affects the chemistry of propagation and thus monomer incorporation into the growing polymer chain.
Industry has generally accepted widespread use of a slurry polymerization process to produce butyl rubber, polyisobutylene, etc. in methyl chloride as diluent. Typically, the polymerization process is carried out at low temperatures, generally lower than −90° C. Methyl chloride is employed for a variety of reasons, including that it dissolves the monomers and aluminum chloride catalyst but not the polymer product. Methyl chloride also has suitable freezing and boiling points to permit, respectively, low temperature polymerization and effective separation from the polymer and unreacted monomers.
The slurry polymerization process in methyl chloride offers a number of additional advantages in that a polymer concentration of up to 40 wt.-% in the reaction mixture can be achieved, as opposed to a polymer concentration of typically at maximum 20 wt.-% in solution polymerizations. An acceptable relatively low viscosity of the polymerization mass is obtained enabling the heat of polymerization to be removed more effectively by surface heat exchange. Slurry polymerization processes in methyl chloride are used in the production of high molecular weight polyisobutylene and isobutylene-isoprene butyl rubber polymers.
In a butyl rubber slurry polymerization, the reaction mixture typically comprises the butyl rubber, diluent, residual monomers and initiator residues. This mixture is either batchwise or more commonly in industry continuously transferred into a vessel with water containing                an anti-agglomerant which for all existing commercial grades today is a fatty acid salt of a multivalent metal ion, in particular either calcium stearate or zinc stearate in order to form and preserve butyl rubber particles, which are more often referred to as “butyl rubber crumb”        and optionally but preferably a stopper which is typically an aqueous sodium hydroxide solution to neutralize initiator residues.        
The water in this vessel is typically steam heated to remove and recover diluent and unreacted monomers.
As a result thereof a slurry of butyl rubber particles is obtained which is then subjected to dewatering to isolate butyl rubber particles. The isolated butyl rubber particles are then dried, baled and packed for delivery.
The anti-agglomerant ensures that in the process steps described above the butyl rubber particles stay suspended and show a reduced tendency to agglomerate.
In the absence of an anti-agglomerant the naturally high adhesion of butyl rubber would lead to rapid formation of a non-dispersed mass of rubber in the process water, plugging the process. In addition to particle formation, sufficient anti-agglomerant must be added to delay the natural tendancy of the formed butyl rubber particles to agglomerate during the stripping process, which leads to fouling and plugging of the process.
The anti-agglomerants in particular calcium and zinc stearates function as a physical-mechanical barrier to limit the close contact and adhesion of butyl rubber particles.
The physical properties required of these anti-agglomerants are a very low solubility in water which is typically below 20 mg per liter under standard conditions, sufficient mechanical stability to maintain an effective barrier, and the ability to be later processed and mixed with the butyl rubber to allow finishing and drying.
The fundamental disadvantage of fatty acid salts of a mono- or multivalent metal ion, in particular sodium, potassium, calcium or zinc stearate or palmitate is the high loadings required to achieve sufficient anti-agglomeration effects. This is a result of the need to form a contiguous surface coating that provides the physical mechanical barrier. At these high levels of anti-agglomerant loadings, issues with turbidity, optical appearance and high ash content of the resulting polymer become a problem in subsequent applications such as sealants and adhesives.
A variety of other elastomers either obtained after polymerization or after post-polymerization modification in organic solution or slurry are typically subjected to an aqueous workup where the same problems apply as well.
Therefore, there is still a need for providing a process for the preparation of elastomer particles in aqueous media having reduced or low tendency of agglomeration.